1. Field of the Invention
The invention relates to a printhead used in equipment for producing black and colour images on a printing medium, generally though not exclusively a sheet of paper, using the thermal ink jet technology, and to a device and associated method of operation for regulating the energy supplied to the emission resistors of the head.
2. Related Technological Art
Equipment of the type described above is known in the art, such as for example printers, photocopying machines, facsimile machines, etc., and particularly printers used for the printing of a document, by way of printing means generally taking the form of fixed or interchangeable printheads.
The composition and general method of operation of a thermal ink jet printer, as also those of the relative ink jet printhead, are already widely known in the art and will not therefore be described in detail here, a more detailed description being provided only of some of the characteristics of the heads that help give a better understanding of the present invention.
A typical ink jet printer schematically consists of:
a system, selectively activated by a motor, for supplying and feeding the sheet of paper whereon the image is to be printed, in such a way that the feeding is performed in a determined direction in discrete steps (line feed),
a movable carriage, sliding on ways in a direction perpendicular to that of the sheet feeding, selectively activated by a motor to perform a forward motion and a backward motion over the entire width of the sheet,
printing means, generally for example a printhead, removably attached to the carriage, comprising a plurality of emission resistors deposited on a substrate (usually a silicon wafer), and disposed inside emission cells or chambers filled with ink, each individually connected to a corresponding plurality of nozzles, through which the head can emit droplets of ink, and to a main tank containing the ink,
an electronic controller which, on the basis of the information received from a computer whereto it is connected and of the presetings made by the user, selectively commands the above-mentioned motors and also the printhead, resulting in the emission therein, by way of the selective heating of the resistors, of the droplets of ink against the surface of the sheet, thus generating a visible image.
According to a recent evolution of the known art, the printheads also comprise, in addition to the emission resistors, the active driving components that selectively supply the energy for heating the emission resistors, generally in the form of MOS transistors integrated within the semiconductor substrate, i.e. produced using known techniques of the silicon wafer integrated semiconductor circuit technology.
From the electrical point of view, these integrated driving components, as they all have substantially identical geometrical and electrical properties, and their associated emission resistors, are typically laid out according to working arrangements known in the sector art in a matrix of rows and columns, so as to minimize the number of connections and contacts between the head and the electronic controller.
The energy is supplied by the MOS transistors to the emission resistors, selectively enabling a current supplied by a voltage power supply unit to flow through the said resistors, all the emission resistors being connected to this power supply unit. Inside the emission resistor, this current is transformed into thermal energy by the Joule effect, resulting in its heating rapidly to a temperature of more than 300xc2x0 C. A first portion of this thermal energy is transferred to the ink present in the emission chamber surrounding the resistor, vaporizing it with the resultant enucleation of a vapour bubble and thus causing the expulsion of a droplet of given volume through the nozzle connected to that emission chamber. A second portion of this thermal energy is lost by conduction through the common substrate (the silicon wafer) whereupon are deposited the emission resistors, increasing the temperature Ts of the substrate and thus of the head as a whole and of the ink contained therein, with respect to the ambient temperature.
The phenomenon of droplet emission may be better understood with reference to the graph in FIG. 1, illustrating the experimentally proven trend represented by the curve 30 of the volume VOL of the droplet of ink emitted by a nozzle, in relation to the thermal energy E supplied to the emission resistor in the cell connected to the nozzle, for a given constant value of the temperature Ts of the substrate.
As shown by the graph, below a value Es (threshold energy) the droplet is not formed, since the resistor does not reach a high enough temperature to vaporize the ink surrounding it. If the energy E supplied to the resistor is increased from value Es to value Eg (knee energy), the volume VOL of the droplets emitted increases substantially proportionally to the increase in energy E supplied to the emission resistor; beyond the value Eg, the volume VOL on the other hand remains substantially unchanged with the increase in the energy E supplied to the resistor. This zone is the zone normally used as the working zone.
The knee energy Eg of a thermal ink jet head is a characteristic of the geometrical and manufacturing configuration adopted, apart from being also dependent on the working temperature TS of the substrate (Si wafer), as seen above. With all other conditions being equal, it varies from head to head as a result of deviations entering the manufacturing processes. In particular, for the heads with integrated driving components, it depends largely on the following parameters typical of the manufacturing process:
thickness of the field oxide SiO2 (Locosxe2x80x94local oxidation of the Silicon substrate),
thickness of the protective passivation (BPSGxe2x80x94Boron/Phosphorous silicon glassivation),
thickness of the SiN and SiC protective layers on the emission resistors,
thickness of the Ta anti-cavitation layer,
resistance value and geometrical dimensions of the emission resistors,
the RON value of the integrated MOS active drive components.
Use is made of the asymptotic characteristic of the pattern of the volume VOL of the droplets in relation to the energy E supplied to the emission resistor in defining the typical working value EI for the energy E to be supplied to the emission resistor (energy operating point). In current practice, for example, a value is taken for EI that is considerably higher than Eg, so that any limited fluctuations of the thermal energy E supplied to the emission resistor (for various reasons, for example the natural tolerances of the power supply voltage value and of the duration of the current pulse supplied to the emission resistors by the printer the head is fitted on) or deviations of the value Eg due to the tolerances of the head manufacturing parameters, do not entail significant variations of the volume VOL of the droplets emitted.
This is due to the fact that the energy operating point of the emission resistors is in any case inside the asymptotic portion of the curve 30, thereby avoiding the occurrence of unstable operating conditions, which could on the other hand occur if EI were to drop below Eg and the droplet volume were to become variable.
However, use of a value of EI that is considerably higher than Eg also involves a series of negative effects, on account of the rise in temperature of the head due to the portion of thermal energy that is not used for emission of the droplet of ink. Among these negative effects are:
the volume of the droplets of ink emitted by the nozzles, for a like working energy value EI, increases with the rise in temperature of the substrate (and therefore of the ink) causing, as illustrated above, a corresponding variation of the diameter of the elementary dots printed on the paper and uniformity of the printout deteriorates accordingly. The phenomenon may be so marked that the characters printed at the top of a page may differ significantly in optical density from those printed at the bottom, due to the rise in head temperature caused in printing the page;
furthermore, the reaching of very high head temperature levels on certain specific emission resistors activated frequently during printing may lead to a phenomenon of deposition of carbon residues following decomposition of the ink on the resistor, dramatically reducing the working lifetime of the resistor and causing operating anomalies of the printhead due to failure of the relevant nozzle to emit ink.
To combat these negative effects at least in part, methods and devices have been proposed in the known art with the essential objective of stabilizing the temperature Ts of the substrate, in other words of having the head work at a substantially constant substrate temperature Ts.
For example, one suggestion was to reduce the printing speed (and thereby to reduce the droplet emission frequency) when the temperature Ts tends to exceed a defined limit, in order to increase the time allowed the head to cool naturally and stabilize at a lower temperature. Another was to interrupt printing when the substrate temperature exceeds a predetermined level. These solutions are, however, unsatisfactory because they are detrimental to the work speed or throughput, a requirement that is constantly more and more appreciated among users of ink jet printers.
Systems have also been suggested for maintaining the substrate temperature Ts constant, by making the head work permanently at an established maximum temperature level using, for example, either supplementary resistors in addition to the emission resistors to heat the head, if and as necessary; or by using the emission resistors themselves to heat the head. In this case, the emission resistors of the nozzles not required to emit droplets of ink are still heated, but with energy pulses of too high a frequency to produce droplet emission. Both these solutions, however, require that the head be fitted with a temperature sensor, in the form of a thermistor, for instance, fitted in contact with the head, duly making the head more complex and more costly to build. Nor are these solutions entirely satisfactory because they fail to solve the problem of the carbon deposits on the emission resistors, since stabilization of the temperature takes place at high levels.
Accordingly it is preferred to adopt a different strategy, consisting in controlling the working energy EI supplied to the emission resistors, so as to supply each head fitted on the printer an energy that is only slightly greater than the effective energy Eg characteristic of that specific head; since however, as already seen, the value of Eg varies from head to head, this value must be known beforehand or, alternatively, the printer employed must dispose of means for measuring a characteristic of the emission resistor of the head, used as the basis for definition of the correct driving conditions for the head fitted on that printer.
One example of a solution is that described in the European Patent Application EP 626266 for a head comprising a xe2x80x9cdummyxe2x80x9d emission resistor, i.e. one not used for generating droplets of ink, but having exactly the same characteristics, resistance in particular, as the emission resistors, being manufactured in the same process and with the same parameters as the emission resistors. Depending on the value of this dummy resistor as measured at the end of the head manufacturing process, the heads are divided into classes corresponding to established resistance ranges, each head then being given a code in relation to its class and the code being recognized by the printer the head is fitted on, for correct adaptation of the current supplied to the emission resistor.
This system, however, apart from the fact that its precision decreases the wider the interval assigned to each subdivision of the head resistance variability range, does not make any allowance for other manufacturing factors that also contribute to differentiating between heads which may have resistance values in the same class, such as for example, thickness of the insulating layer separating the resistor from the ink, nor does it make any allowance for different power supply voltages of the various printers that a head may be fitted on.
The latter problem has been solved, for example, by the U.S. Pat. No. 5,083,137 in which the power supply voltage of the emission resistors provided by the printer is variable, and can be regulated by way of a counter-feedback circuit in relation to the signal provided by comparing means that compare the voltage actually supplied to the emission resistors with a predetermined rekerence value.
The European Patent Application EP 752313 A discloses a solution in which the temperature of the substrate of the printhead is maintained constant by means of a feedback circuit that regulates the power to be dissipated by an additional resistor.
However, it will be clear that, even if the teachings of all the previously known solutions are applied simultaneously, the problem would still not be fully resolved of supplying each head a working energy EI only slightly greater than the knee energy Eg characteristic of that specific head.
The object of this invention is to define a device for controlling the energy supplied to an emission resistor of a thermal ink jet printhead fitted on a printer, said emission resistor being capable of generating a vapour bubble upon reaching an enucleation temperature, and said printer comprising means for supplying a variable amount of said energy to said emission resistor, characterized in that it comprises means integrated on said head for detecting said enucleation temperature, and means for regulating said variable amount of said energy supplied to said emission resistor, so that said emission resistor reaches said enucleation temperature, dependent on said means for detecting said enucleation temperature.
In this way, compensation is provided for all the variables and deviations of the printhead manufacturing process, and for the different driving characteristics of the various printers that the head can be fitted on.
Another object of the invention is to define a method for controlling the energy supplied to an emission resistor of a thermal ink jet printhead fitted on a printer, said emission resistor being capable of generating a vapour bubble upon. reaching an enucleation temperature, and said printer comprising means for supplying a variable amount of said energy to said emission resistor, characterized in that it comprises the following steps: having means integrated on said head for detecting said enucleation temperature; regulating said variable amount of said energy supplied to said emission resistor, so that said emission resistor reaches said enucleation temperature, dependent on said means for detecting said enucleation temperature.
Another object of the invention is to define a method for controlling the energy supplied to an emission resistor of a thermal ink jet printhead fitted on a printer, said emission resistor being capable of generating a vapour bubble upon reaching an enucleation temperature, and said printer comprising means for supplying a variable amount of said energy to said emission resistor, characterized in that it comprises the following steps: having means integrated on said head for detecting a variation of the positive temperature coefficient of the resistance upon reaching said enucleation temperature; regulating said variable amount of said energy supplied to said emission resistor, so that said emission resistor reaches said enucleation temperature, dependent on said means for detecting said variation of the positive temperature coefficient of the resistance.
A further object of the invention is to define a thermal ink jet printhead comprising means for supplying a variable amount of energy to an emission resistor capable of generating a vapour bubble upon reaching an enucleation temperature, characterized in that it further comprises means for detecting said enucleation temperature comprising a first resistor obtained from a layer of electrically conductive material in correspondence with a test resistor, identical in construction to said emission resistor.
Yet another object of the invention is to define an ink jet printer comprising a thermal printhead comprising means for supplying a variable amount of energy to an emission resistor capable of generating a vapour bubble upon reaching an enucleation temperature, characterized in that said printhead further comprises means for detecting said enucleation temperature comprising a first resistor obtained from a layer of electrically conductive material in correspondence with a test resistor identical in construction to said emission resistor.
The above objects are fulfilled by a device for controlling the energy supplied to an emission resistor of a thermal ink jet printhead, the associated method of operation, the associated printhead and associated printer, characterized as defined in the main claims.
A clearer understanding of these and other objects, characteristics and advantages of the invention will be gained from the following description of a preferred embodiment, provided purely by way of an illustrative, non-restrictive example, with reference to the accompanying drawings.